The purposes of the iinvestigation are to study of the mechanism of action of the IUD and the prolonged effect of the devices (polyethylene and silk thread). Our data indicate that the neutrophilicresponse in the uterus induced by the IUD may have an important role in preventing implantation of the blastocysts. This hypothesis will be expanded in two studies. The first is to observe the effect of a leukopenic drug on the influx of neutrophils following IUD insertionand on fetal development. The second is to observe implantation and fetal development in uteri treated with anknown number of neutrophils. At the presnt time the data on the long-term effect have to be evaluated.